TypeScript Coach

Parametrized types

Functions can have parametrized types that are .

TODO: syntax diagram

// Returns the default value if value is null
function getOrDefault<AnyType>(value: AnyType | null, defaultValue: AnyType): AnyType {
    if (value == null)
        return defaultValue
    else
        return value
}
const safeString = getOrDefault<string>(null, '123')
const invalid = getOrDefault(123, '123')

The parametrized type can be restricted to extend a specific enclosing type.

function getOrDefault<T extends string | number>(value: T | null | undefined, defaultValue: T): T {
    return value == null ? defaultValue : value
}